If the 555 is anything like the 595, then I couldn't imagine needing an amp. The 595's require less volume than my iPod and OMX-80's.
IIRC, it has more to do with something called an impedance rating than the physical size of the phones. Also, cheaper phones seem to require more volume because you have to turn them up higher to reproduce details.
However, I would avoid the HD555/595's, for comfort reasons. Get something lighter, with more comfortable support. The headband really hurt my head until I modded it with some additional foam, but it looks ugly that way. And they still mess my hair up terribly, so bad that I have to wash it right after listening or walk around with my head looking like a molded packing insert.
The OMX-80's I could wear all day, but they may be a bit below your desired budget/quality. The flex tips are perfect, though: You can bend them any way you like -- around the ear, back around your head, etc. -- which works well for people like me who wear glasses.
I've worn IEMs (Shure E4c's w/ the optional triple flange tips ?) all day before, and my ears hurt so badly from shoving the tips in/out that I just couldn't wear them anymore. Nice sound, though. They doubled as hearing protection, too. Sweat (from wearing while working outside) eventually killed them. Maybe I just got a bad model (comfort-wise), but I would seriously research the comfort level of any IEMs you are considering.
Also, getting IEMs to seal properly (for sound quality) can be a total b**** if the tips you get aren't well designed and/or can't fit your ears. You also have to worry about wax, just from normal usage. (I'm assuming good ear hygiene). Foam tips need to be replaced regularly; plastic tips can be removed and washed. Plastic tip discoloration can be fixed by placing them out in the sun for a few hours.